The term “fuse” is used in the art to identify a device that initially (before) has low impedance and, when exposed to a current above a predetermined threshold, switches to a high impedance final (after) state. In its simplest embodiment, a small section of conductor melts when exposed to excess current, thereby interrupting the circuit of which it is a part. The term “antifuse” is used in the art to describe a device whose behavior is substantially the opposite of a fuse, that is, it initially (before) has high impedance and, when exposed to a voltage above a predetermined threshold, switches to a low impedance final (after) state. In its simplest embodiment, a thin insulating dielectric sandwiched between two conductors breaks down when exposed to a “programming” voltage above a predetermined threshold, thereby forming a conductive path through the dielectric so that the device thereafter exhibits much lower final impedance. With an antifuse, it is not necessary that the initial and final impedances be infinity and zero, respectively, only that they are substantially different. Accordingly, as used herein with respect to antifuses, the terms “open” or “OFF” and “closed” or “ON” are intended to refer, respectively, to the initial high impedance state and the final lower impedance state and not to imply that these states have infinite and zero impedance.
Antifuses are much used in modern electronics, especially in connection with integrated circuits (ICs), to provide substantially non-volatile memory. For example, an array of antifuses may be programmed during or after manufacture of an electronic circuit to store certain information or commands within the electronic circuit or the memory portion thereof. The stored information may be an identifying serial number or other unique label, or may be the binary code for a software routine, or may determine which parts of a particular circuit are active, or what conversion factor should be used in a calculation or any number of other functions where non-volatile memory or state information is needed.
It is important that the antifuse be easy to manufacture, be reliable in operation, be easily and reliably programmed and consistently sensed, and at the same time be compatible with the other elements that may be part of the same circuit or device, especially when they are manufactured together as for example in an integrated circuit (IC) or other common electronic assembly. It is known to use planar metal-oxide-semiconductor (MOS) structures to form antifuses. These are attractive because they can be formed by the same manufacturing technology used for forming associated metal-oxide-semiconductor-field-effect-transistors (MOSFETs) in complex ICs. However, difficulties still remain in obtaining antifuses of optimum properties using manufacturing technologies compatible with the active devices needed for the IC of which both are a part. Accordingly, there is an ongoing need for improved antifuse structures and manufacturing methods compatible with advanced integrated circuit (IC) technology.